1. Field of the Invention
The present invention relates to a sheet manufacturing method and a manufacturing apparatus of manufacturing a sheet (a plate-like base) from a melt containing a metal material or a semiconductor material. Particularly, the present invention relates to a technique for inexpensively manufacturing solar cells, where a silicon sheet for a solar cell is directly manufactured from a silicon melt. More particularly, the present invention relates to a silicon sheet manufacturing apparatus and method enabling manufacture of a silicon sheet, which has protrusions or curved portions at least on the melt side when viewed in section.
2. Description of the Background Art
Examples of conventional methods of manufacturing polycrystalline silicon wafers for use in solar cells include a method of casting to form a polycrystalline body of e.g., silicon, disclosed in Japanese Patent Laying-Open No. 6-64913. In this method, a high-purity silicon material containing a dopant of phosphorus, boron, or the like, is heated to melt in a crucible placed in an inert ambience. Then, a silicon melt is poured into a casting mold for gradually cooling, so as to provide a polycrystalline ingot. Accordingly, for manufacturing polycrystalline silicon wafers for use in solar cells from thus obtained polycrystalline ingot, the ingot is to be sliced by a wire saw or inner diameter blade.
A method of continuously casting a silicon plate, disclosed in Japanese Patent Laying-Open No. 7-256624, involves manufacture of a silicon sheet without slicing. In this method, a silicon melt is poured into a horizontal heat casting mold and a dummy graphite plate is horizontally inserted such that its leading end is dipped into the silicon melt under a control plate. When the silicon adheres to the leading end of the graphite plate, the silicon plate is horizontally pulled out with use of a roller. Coolant gas supplied from gas blow-off pipe of a cooling apparatus provides for continuous formation of the silicon plate.
Another method of manufacturing a silicon sheet uses a manufacturing apparatus for a silicon ribbon disclosed in Japanese Patent Laying-Open No. 10-29895. The manufacturing apparatus for a silicon ribbon generally has a portion of heating to melt silicon and a cooling roller of a heat-resistant material. The cooling roller, with one end of carbon net wound thereon, is directly dipped into a silicon melt for forming a silicon ribbon on the surface of the cooling roller. The carbon net wound on the cooling roller is pulled with rotation of the cooling roller for rolling out thus formed silicon ribbon. As such, the manufacturing apparatus allows the silicon ribbon, formed from the silicon that first adhered to the carbon net, to be continuously rolled out.
However, the above described conventional methods or apparatuses of manufacturing a silicon plate or a silicon sheet suffer from the following problems. The method of casting a crystalline body of e.g. silicon disclosed in the aforementioned laid-open application No. 6-64913 requires slicing of the polycrystalline ingot, whereby a slicing loss is caused corresponding to a thickness of the wire saw or inner diameter blade. This results in yield decrease and it becomes difficult to provide wafers at low cost.
The method of continuously casting the silicon plate disclosed in the aforementioned laid-open application No. 7-256624 controls the thickness of the silicon plate by pulling out the silicon plate under the thickness control plate. In this case, it is difficult to control a thickness of 600 xcexcm or smaller as is employed for solar cells.
In the method of manufacturing the silicon ribbon disclosed in the aforementioned laid-open application No. 10-29895, the silicon ribbon formed from the silicon that first adhered to the carbon net is continuously pulled and rolled out with rotation of the cooling roller. However, the silicon ribbon is somewhat fragile due to reaction of the carbon net and the silicon. If the formed silicon is extremely thin, the silicon ribbon may break to fall during pulling operation. In this situation, the operation must be stopped and productivity decreases.
Further, a mechanism is provided which pressurizes to apply the silicon melt onto the peripheral surface of the cooling roller by jet pressure. Since the pressure is exerted by agitation of the silicon melt, unwanted pressure may be exerted to the formed silicon, thereby causing defects.
A growth rate of silicon is determined by a number of factors including a temperature of a heater for maintaining the silicon in a molten state, dipping depth, type and flow rate of coolant gas circulating in the cooling roller, rotation speed of the cooling roller and the like. Thus, it is technically difficult to stably pull out the silicon ribbon while controlling the growth rate.
Further, a wedge is provided for removing any silicon residue left on the surface of the cooling roller. Such a wedge is brought into direct contact with the surface of the cooling roller where silicon grows, thereby disadvantageously scratching the surface of the cooling roller or striping a remover applied thereon. Consequently, evenness of the silicon ribbon is impaired.
An inexpensive solar cell requires a base which has excellent evenness and well-controlled thickness and which saves a slicing loss. In any of these conventional cases, it is difficult to provide a thin base with excellent evenness manufactured by mass production at low cost.
An object of the present invention is to overcome the aforementioned problems associated with the conventional technique, so as to provide a method of manufacturing an inexpensive sheet with a desired thickness and excellent uniformity and evenness. The method enables manufacture of the silicon sheet with a desired thickness and excellent evenness without slicing while ensuring productivity.
According to the present invention, the above mentioned object is achieved in the following manner.
According to one aspect of the present invention, a sheet manufacturing method is provided in which a base with protrusions is cooled and the surfaces of the protrusions of the cooled base are dipped into a melt material containing at least one of metal and semiconductor materials to form crystals of the material on the surfaces of the protrusions. Thus, a sheet of the material is produced.
According to another aspect of the present invention, a sheet manufacturing method is provided in which a roller having on its peripheral surface protrusions as well as a cooling system for cooling the protrusions is rotated and the surfaces of the cooled protrusions are dipped into a melt material containing at least one of metal and semiconductor materials to form crystals of the material on the surfaces of the protrusions. Thus, a sheet formed of the material is produced.
According to still another aspect of the present invention, in the sheet manufacturing method, the protrusions include at least one of a dot-like protrusion or a linear protrusion.
According to still another aspect of the present invention, in the sheet manufacturing method, the protrusions include at least one of a dot-like protrusion and a linear protrusion in addition to a planar protrusion.
According to still another aspect of the present invention, in the sheet manufacturing method, the protrusions are coated with a material of at least one of silicon carbide, boron nitride, silicon nitride, and pyrolitic carbon.
According to still another aspect of the present invention, in the sheet manufacturing method, the crystals of the material grow from the protrusions.
According to still another aspect of the present invention, a sheet is provided which has curved portions formed by cooling a base with protrusions, dipping the surfaces of the protrusions of the cooled base into a melt material containing at least one of metal and semiconductor materials, and forming crystals of the material in a curved shape on the surface of the base from the protrusions.
According to still another aspect of the present invention, a sheet is provided which has curved portions and planar portions formed by cooling a base with protrusions including at least one of a dot-like protrusion and a linear protrusion in addition to a planar protrusion, dipping the surfaces of the protrusions of the cooled base into a melt material containing at least one of metal and semiconductor materials, forming crystals of the material on the surface of the base in a curved shape from the dot-like protrusion or linear protrusion, and forming crystals of the material on the surface of the base in a planar shape from the planar protrusion.
According to still another aspect of the present invention, a sheet manufacturing apparatus is provided which includes: a roller having on its peripheral surface protrusions and a cooling system for cooling the protrusions; and a crucible into which includes a material containing at least one of metal and semiconductor material and which allows the protrusions to be dipped into the melt by rotation of the roller.
According to still another aspect of the present invention, a solar cell is provided by forming an electrode on a sheet having a curved portion formed by cooling a base with protrusions, dipping the surfaces of the protrusions of the cooled base into a material containing at least one of a metal material and a semiconductor material, and forming crystals of the material on the surface of the base in a curved shape from the protrusion.
According to still another aspect of the present invention, a solar cell is provided by forming an electrode on a planar portion of a sheet with a curved portion and a planar portion obtained by cooling a base with protrusions including at least one of dot-like protrusions and linear protrusions in addition to planar protrusions, dipping the surfaces of the protrusions of the cooled base into the melt material containing at least one of a metal material and a semiconductor material, forming crystals of the material on the surface of the base in a curved shape from the dot-like protrusion or the linear protrusion, and forming crystals of the material on the surface of the base in a planar shape from the planar protrusion.
According to still another aspect of the present invention, a silicon sheet manufacturing apparatus is provided for manufacturing a silicon sheet by rotating a cooling roller to solidify and form a silicon sheet. The apparatus is characterized in that the surface of the cooling roller is provided with protrusions arranged in a dot-like pattern or a linear pattern when viewed from above.
According to still another aspect of the present invention, the silicon sheet manufacturing apparatus is characterized in that spaces between the protrusions are in a V or U like shape (such spaces are hereinafter referred to as V or U grooves).
According to still another aspect of the present invention, the silicon sheet manufacturing apparatus is characterized in that the surface of the cooling roller is coated with SiC.
According to still another aspect of the present invention, the silicon sheet manufacturing apparatus is characterized in that the pitch of the V or U groove is at least 0.05 mm and at most 5 mm.
According to still another aspect of the present invention, the silicon sheet manufacturing apparatus is characterized in that the height of the protrusions is at least 0.05 mm and at most 5 mm.
According to still another aspect of the present invention, a silicon sheet manufacturing method is provided for manufacturing a silicon sheet by rotating a cooling roller to solidify and form crystals of a silicon melt. The method is characterized in that the crystals are solidified and formed from protrusions of the cooling roller arranged in a dot-like pattern or a linear pattern when viewed from above.
According to still another aspect of the present invention, a solar cell is provided by forming an electrode on a silicon sheet obtained by rotating a roller having on its peripheral surface protrusions including at least one of a dot-like protrusion and a linear protrusion as well as a cooling portion for cooling the protrusions, dipping the surfaces of the cooled protrusions, and forming silicon crystals on the surfaces of the protrusions.
For producing a silicon sheet, the surface of the cooling roller for forming the silicon sheet has protrusions arranged in a dot-like pattern or a linear pattern and a silicon melt is solidified at the protrusions, so that curved protrusions are formed at least on one side of the silicon sheet. The silicon sheet of the present invention is characterized by a curved surface formed on the side of the silicon melt. The continuous protrusions serve as ribs to save the silicon material used and to enable mass production of silicon sheets with reduced thickness and yet sufficient strength.
The silicon sheet manufacturing apparatus according to the present invention may include: a roller having on its peripheral surface protrusions including at least one of a dot-like protrusion and a linear protrusion as well as a cooling portion for cooling the protrusions; and a crucible containing a silicon melt and capable of dipping the protrusions into the silicon melt by rotation of the roller.
The cooling roller is characterized in that the spaces between the protrusions on the surface of the cooling roller dipped into the silicon melt are V or U grooves. The protrusions with V or U grooves are formed in the surface of the cooling roller in a dot-like pattern or in a linear pattern with respect to a rotation direction. The top portions of the protrusions are dipped into the silicon melt, which are in turn cooled from inside. This produces crystal nuclei of silicon at the top portions of the protrusions, which gradually grow and combine together, i.e., one nuclei combines with those growing from the adjacent top portions. Thus, a silicon sheet is produced. In this way, stable growth of the silicon sheet is enabled.
Further, use of the cooling roller having the V or U groove-structure enables manufacture of a corrugation or wave-like silicon sheet directly on the cooling roller, depending on manufacturing conditions of the silicon sheet. Accordingly, the sheet has curved portions not only on the melt side but also on the side of the cooling roller, in accordance with the above mentioned dot-like pattern or linear pattern. This is due to changes in shape of the silicon crystals growing from peaks of the V or U grooves. Such curved portions provide for significant reduction in silicon material used.
In addition, the following method further facilitates industrialization. A silicon carbide (SiC) coating is applied to the surface of the cooling roller. According to the above mentioned structure, the presence of the SiC film on the surface of the cooling roller prevents contamination by the material used for the cooling roller. The pitch of the V or U grooves of the cooling roller is at least 0.05 mm and at most 5 mm. According to the structure, the crystal nuclei of the silicon are produced only at the peaks of the V or U grooves of the cooling roller, so that the grain size or thickness of the silicon sheet can readily be controlled.
In the structure, although a pitch less than 0.05 mm may increase the number of protrusions to be dipped into the silicon melt to achieve higher growth rate, it is not desirable since the grain size would become extremely small. Although a pitch greater than 5 mm may provide a greater grain size, in this case, a problem associated with the growth rate arises.
In the structure, the height of the protrusions of the cooling roller, i.e., the depth of the V or U grooves is desirably at least 0.05 mm and at most 5 mm.
In the structure, only the peaks of V or U grooves of the cooling roller can be reliably dipped into the silicon melt in a portion for heating and melting silicon. The depth smaller than 0.05 mm may disadvantageously cause troughs of the V or U grooves of the cooling roller to be dipped into the silicon melt. As a result, a contact area increases and removability is impaired.
As in the foregoing, by controlling the surface structure of the cooling roller and properly setting a dipping condition or the like, a silicon sheet with a moderate thickness is produced.
The silicon sheet manufacturing apparatus further includes a bar-like remover for a silicon sheet, which is disposed at the trough of the cooling roller.
In the structure, the crystal nuclei of silicon are produced at the peaks of V or U grooves of the cooling roller, which gradually grow to combine with those from adjacent peaks to form a silicon sheet. Thus formed silicon sheet is readily removed as a single sheet from the V or U grooves of the cooling roller.
In this case, the remover is disposed at the trough of the cooling roller. Accordingly, the remover would not cause any damage to the peaks of the V or U grooves at which crystals grow. In addition, the presence of the SiC coating on the cooling roller can prevent a substance at the trough from falling.